This question is driving me crazy, and although there's a fair amount of general discussion out there on brett/bacteria and head space, I'd really appreciate some input that applies to my situation.

Based on my kettle size and stovetop heat source, all of my brews result in 4 gallons in a 5 gallon fermentor. I typically lose 1/2 gallon to trub, and net 3.5 gallons in bottles. Right now, I'm planning to brew a sour brown ale with ECY01. Based on my experience, I'll have 3.5 gallons eligible to go into secondary for a year's aging.

I only have two fermentors, both glass, both 5 gallons. If I rack my 3.5 gallons of sour to a 5 gallon carboy, that's a full 1.5 gallons of head space. Is that too much? Am I risking vinegar, even if I behave myself and keep the airlock topped off and never take samples?

The alternative doesn't sound very attractive--obtaining a 3 gallon better bottle for secondary, and either having NO head space, or else racking only 2.5 gallons over and losing some precious sour beer.

I'm willing to explore topping off with water or wort, but for the moment, let's discount that option. If you had to choose between the above two options (5 gallon glass secondary or 3 gallon better bottle) which would you choose, and how much or how little head space would you tolerate?

I'm thinking about this a bit more, and the panic is subsiding. I think the answer to my problem may be a lb. of DME, which would help me to maintain gravity while bumping up volume by 1 gallon in my 5 gallon glass secondary.

I'm inclined to add that gallon when racking my sour to secondary. Is there a compelling reason to add it to primary--i.e., put the DME in the boil on brew day, and dilute to my intended OG in the fermenter? It seems like there might be considerations to take into account when feeding a mixed fermentation, that wouldn't apply in a clean fermentation.

Any thoughts on any and all of the above questions? If I just need to RDWHAHB, please let me know.

As long as you don't shake up the fermentor, keep the airlock topped up and don't sample too much, I would not fear a five gallon fermentor with 3.5 gallons inside. That will make some people nervous about acetic acid production.

If you feel uncomfortable about having that much headspace, why not just break it up between a three gallon better bottle and a gallon jug? I have aged quite a bit of sour beer in a wine jug without problem. I have three year old lambic in one now. That would give you four gallons of space, which gives you about a quarter gallon of headspace in each. It also gives you the flexibility to bottle some early or hold on to some for a longer period of time.

Thanks once again for your help, RAM. I have a few gallon jugs, so that is an option. However, at this point, I think I'm leaning toward increasing my batch volume so that I can end up with ~4.5 gallons in my 5 gallon secondary.

In addition to my two 5 gallon glass carboys, I also have a 5.5 gallon carboy. I didn't mention it before because as I understood my dilemma, it was to figure our how to step DOWN in volume for secondary fermentation. I didn't see where the bigger carboy fit in the picture.

I haven't used my 5.5 gallon carboy since I went AG and stopped doing partial boil extract batches. But I think it's time to dust it off. It probably doesn't make sense for me to buy a 3 gallon better bottle when, apart from this sour, I *never* use a secondary. I already have the 5.5 gallon carboy and two 5 gallon carboys for primary / secondary, and $4 for a pound of DME completes the picture.